1. Field of the Invention
The present invention relates to printers in which magnetic print hammer actuators associated with different hammer springs and containing hammer drive coils have drive transistors coupled to energize the hammer drive coils to provide selected firing of the hammer springs to effect printing, and more particularly to circuits for detecting fault conditions within the magnetic print hammer actuators and including particularly the drive transistors thereof.
2. History of the Prior Art
Printers are known in which hammer drive coils within magnetic print hammer actuators are selectively energized to provide release or firing of associated hammer springs to effect impact printing. An example of such a printer is provided by U.S. Pat. No. 3,941,051 of Barrus et al., PRINTER SYSTEM, which patent issued Mar. 2, 1976 and is commonly assigned with the present application.
In the printer described in the Barrus et al. patent, each of a plurality of hammer springs mounted along the length of an elongated hammerbank is normally held in a retracted position by a different magnetic print hammer actuator associated therewith. Each magnetic print hammer actuator includes a permanent magnet and a pole piece which form a portion of a magnetic path together with the hammer spring. An upper end of the hammer spring is held against the pole piece by the permanent magnet when in the retracted position. Each magnetic print hammer actuator also includes a magnetic coil mounted on the pole piece adjacent the free upper end of the associated hammer spring. Momentary energization of the magnetic coil overcomes the effects of the permanent magnet and causes the upper free end of the hammer spring to fly away from the pole piece so that a dot printing impact tip mounted thereon impacts a length of ink ribbon against a print paper. The hammer spring then rebounds back into the retracted position against the pole piece where it is held by the permanent magnet in preparation for tee next release or firing of the hammer spring by energizing the magnetic coil.
In printers of the type described in the Barrus et al. patent, the various magnetic hammer drive coils which are mounted on pole pieces along the length of the hammerbank so as to form a part of the hammerbank are coupled by wire busses to circuits located adjacent to but separate from the reciprocating hammerbank within the printer. The circuits include a plurality of drive transistors, each of which is normally switched off and is coupled to be switched on momentarily so as to energize an associated one of the hammer drive coils on the hammerbank when the hammer spring associated with that coil is to be fired. As is true of practically any electrical circuit, the drive transistors are not without their occasional reliability problems. Occasionally, one or more of the drive transistors will begin leaking and then eventually short circuit when switched off, or they will assume an open circuit condition and fail to properly energize the associated magnetic coil when switched on. The magnetic coils themselves will occasionally short or become open circuited.
Because of the occasional failures that may occur with the drive transistors or the hammer drive coils, it has become common practice to provide printers of this type with a detection circuit in an attempt to detect and signal failures of the drive transistors or the hammer drive coils. An example of such a detection circuit is provided by U.S. Pat. No. 4,405,922 of Nishino et al., FAILURE DETECTION CIRCUIT FOR IMPACT PRINTERS OR THE LIKE, which patent issued Sept. 20, 1983. The Nishino et al. patent describes a detection circuit in which the drive transistors and the associated hammer drive coils of the printer may be tested either during actual printing or when the printer is simply turned on but is not printing. As each drive transistor is turned on, a signal condition within a circuit associated therewith is sensed and then passed to a logic circuit together with information indicating that switching of the transistor has occurred. If a drive transistor has been switched on but fails to properly energize the associated hammer drive coil to provide hammer release, this error condition is signalled by the circuit.
Detection circuits such as that described in the Nishino et al. patent are generally effective in providing an indication when a component such as a drive transistor or a hammer drive coil has completely failed. However, such circuits do not provide an early warning of component failure which is important in taking quick action to minimize the consequential damage and losses that result therefrom. For example, the failure process for a drive transistor may take several minutes or longer to occur. However, once such transistor shorts out, it is usually less than a minute before the associated hammer drive coil on the hammerbank burns out. While the drive transistor typically forms a part of a circuit card which is relatively inexpensive and is easily replaced within the printer, the associated hammer drive coil forms a part of the hammerbank which is quite expensive and which is not very easily replaced. While the hammerbank contains many hammer drive coils, the burnout of merely one coil requires removal and repair of the hammerbank. Repair of the burned out coil is tedious and time-consuming, and the removal or replacement of the hammerbank must usually be done by a trained technician having specialized tooling.
It will therefore be appreciated that early detection of drive transistor failure is highly desirable. At the same time, it would be desirable to provide an improved circuit of relatively simple design for detecting open circuited as well as shorted conditions of the drive transistors as well as open and shorted conditions of the magnetic coils. It would furthermore be desirable to simplify the detection circuitry to the extent that the fault detecting portions thereof can be easily and simply coupled to common junctions within the circuitry of the magnetic print hammer actuators for the hammerbank.